Field emission displays (FEDs) are a type of die assembly used in connection with computers, television sets, camcorder viewfinders, and other electronic devices requiring flat panel displays. FEDs have a baseplate and a faceplate positioned opposite to one another across a narrow vacuum gap. In large FEDs, a number of spacers are positioned between the baseplate and the faceplate to prevent atmospheric pressure from collapsing the plates together. The baseplate typically has a base substrate upon which a number of sharp, cone-shaped emitters are formed, an insulator layer positioned on the substrate having cavities through which the emitters extend, and an extraction grid formed on the insulator layer around the cavities. The faceplate has a substantially transparent substrate, a transparent conductive layer disposed on the inner surface of the transparent substrate, and a cathodoluminescent material deposited on the transparent conductive layer. In operation, a potential is established across the extraction grid and the emitter tips to extricate electrons from the emitter tips. The electrons pass through the cavities in the insulator layer and the holes in the extraction grid, and impinge upon the cathodoluminescent material in a desired pattern.
Typically, many FED baseplates are fabricated on a single wafer. The baseplates are separated from one another by cutting the wafer in the spaces between the individual baseplates. Wafers are conventionally cut by a thin abrasive wheel that saws through the wafer much like a circular saw cuts wood, or by a laser scribe that scribes a kerf in the wafer which is then broken along the kerf. Abrasive wheels are generally a metal blade embedded with very small diamonds, and thus they produce a significant amount of particulate matter as they cut the wafer. Abrasive wheels cut the wafers at a rate of 1.0 to 1.5 inches per second. Laser scribing also produces some particulate matter because small particles break off of the wafer when it is broken.
Most microelectronic devices are sensitive to particulate matter. FED baseplates are particularly sensitive to particulate matter because they may be ruined by only a few particles. A single particle can obstruct or short out several thousand emitters and thus prevent several pixels of a display from illuminating. Since the human eye can notice when a single pixel on a display does not illuminate, a single particle on a baseplate can effectively rain an entire FED. On the other hand, memory devices are not mined by a few particles because each particle will only affect a small percentage of the available memory which will not be noticed in operation. Accordingly, compared to many other types of microelectronic devices, it is even more important to protect FED baseplates from the particulate matter produced by conventional cutting techniques.
FED baseplates are conventionally protected by depositing a 2-3 .mu.m thick layer of organic protective material over the baseplates before cutting the wafer, and then attaching a thin plastic film to the surface of the organic protective layer. The organic protective layer and plastic film form a barrier that prevents any particulate matter from contacting the features on the baseplates. After the wafer is cut, the plastic film is removed from the organic protective material, and the organic protective material is washed from the baseplates.
One problem with conventional cutting techniques is that the process of protecting FED baseplates from particulate matter increases the production time and cost of manufacturing baseplates. Because the conventional baseplate protection process described above does not add any permanent structural features to the baseplates, the additional materials and process steps are, in effect, useless after the baseplates are finished. Therefore, it would be desirable to minimize the time and cost of protecting the baseplates from particulate matter produced by conventional cutting techniques.
Another problem of conventional cutting techniques is that the removal of the organic protective material itself can damage the emitters or the extraction grid on the baseplate. Still another problem of conventional cutting techniques is that the abrasive wheels create small cracks in the wafer that can grow when the wafer is heated. Accordingly, it would also be desirable to develop a method for cutting a wafer that minimizes the losses associated with protecting the baseplates with an organic protective material, and reduces cracks in the wafer.
One desirable technique for cutting wafers is to completely cut them with a laser. Lasers do not produce any particulate matter because they burn through the silicon and form areas of melted silicon, or slag, around the periphery of the cut pieces. To date, however, completely cutting through an FED baseplate with a laser has not been commercially feasible because the slag may cover the emitters or the contact points and, thus, prevent the baseplate from operating. Therefore, it would be desirable to develop a method to cut a wafer of FED baseplates with a laser that does not produce any measurable slag.